Originally Posted by
Machinbird
Can someone who works on current 737 models provide some information on the actual AOA sensors used?
I can't meet that first requirement but in the absence of any more authoritative answer, I think you are likely correct based on 737NG information published (which matches some NG AMM chapters I might have acquired)
here. Search on that page for "SMYD Analog Int" and you'll find the SMYD diagram where in the top left we see "sin / cos / com" analog inputs from AOA sensor - bingo. Assuming it hasn't changed between NG and MAX.
However, although SMYD implements stick shaker, it doesn't do speed trim, that is the FCC (and they got both). The text beneath that diagram shows that SMYD outputs AOA to data acquisition, but not FCC/ADIRU. FCC gets AOA from ADIRU (where it is used in speed calculation), and as far as I can see ADIRU also gets it direct from the sensor.
Pretty sure (very rusty...) that decoding resolver output for position (not angular velocity) is just a matter of relative voltage magnitude of the sin/cos signals, so wouldn't be surprising to see that duplicated. That means you'd need to have two simultaneous conversion failures. I think, and if that bit is all still same as NG.
Since both right and left sensors appear to have been following the AOA, but the left sensor had a 20 degree offset, the point in the aircraft where the sine-cosine signal pair are combined before digitizing would be where to look for the source of the AOA error. Loss of either of the AC signals would create a stationary AOA reading.
Partial loss, or rather additional impedance in one signal line might do what? I
think it could lead to offset reading, not sure it would be a consistent offset though, been doing software-only for too long.
Broken sensor or misfitted sensor is probably the place to look first - not least because identifying a wiring issue now is going to be tricky...